U.S. patent application number 11/636940 was filed with the patent office on 2007-06-14 for mobile object detection method and detection system.
This patent application is currently assigned to National Institute of Advanced Industrial Science and Technology. Invention is credited to Hideo Itoh, Ryosaku Kaji.
Application Number | 20070132582 11/636940 |
Document ID | / |
Family ID | 38138724 |
Filed Date | 2007-06-14 |
United States Patent
Application |
20070132582 |
Kind Code |
A1 |
Kaji; Ryosaku ; et
al. |
June 14, 2007 |
Mobile object detection method and detection system
Abstract
The present invention provides a mobile object detection method,
and a mobile object detection system, that allow detecting the
movement of an object based on measurements of the electric field
intensity generated by transmitters such as IC tags, while curbing
implementation costs. Movement of the object is detected by
measuring electric field intensity generated by a plurality of
transmitters arranged in a detection space, obtaining a difference
between the electric field intensity and an average value of the
electric field intensity in a state where an object in the
detection space is not moving, and detecting the movement of the
object when the difference is equal to or larger than a threshold
value based on a standard deviation of the electric field intensity
in a state where the object in the detection space is not
moving.
Inventors: |
Kaji; Ryosaku; (Tokyo,
JP) ; Itoh; Hideo; (Tokyo, JP) |
Correspondence
Address: |
WOLF GREENFIELD & SACKS, P.C.
600 ATLANTIC AVENUE
BOSTON
MA
02210-2206
US
|
Assignee: |
National Institute of Advanced
Industrial Science and Technology
Tokyo
JP
|
Family ID: |
38138724 |
Appl. No.: |
11/636940 |
Filed: |
December 11, 2006 |
Current U.S.
Class: |
340/561 |
Current CPC
Class: |
G08B 13/2491
20130101 |
Class at
Publication: |
340/561 |
International
Class: |
G08B 13/26 20060101
G08B013/26 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 13, 2005 |
JP |
2005-358647 |
Claims
1. A mobile object detection method, comprising: measuring electric
field intensity generated by a plurality of transmitters arranged
in a detection space; obtaining a difference between the electric
field intensity and an average value of the electric field
intensity in a state where an object in the detection space is not
moving; and detecting the movement of the object when the
difference is equal to or larger than a threshold value based on a
standard deviation of the electric field intensity in a state where
the object in the detection space is not moving.
2. A mobile object detection system, comprising: means for
measuring electric field intensity generated by a plurality of
transmitters arranged in a detection space; and means for obtaining
a difference between the electric field intensity and an average
value of the electric field intensity in a state where an object in
the detection space is not moving, and detecting the movement of
the object when the difference is equal to or larger than a
threshold value based on a standard deviation of the electric field
intensity in a state where the object in the detection space is not
moving.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] Foreign priority benefits are claimed under 35 U.S.C.
.sctn.119(a)-(d) or 35 U.S.C. .sctn.365(b) of Japanese Application
No. 2005-358647 filed Dec. 13, 2005, which is hereby incorporated
by reference in its entirety.
FIELD
[0002] The present invention relates to a mobile object detection
method and detection system for detecting the movement of an object
based on measurements of electric field intensity generated by a
transmitter such as an IC tag.
BACKGROUND
[0003] Recent years have witnessed a surge in research and
development of applications based on short-range wireless
communication technology using radio waves and/or light in, for
instance, RFID (Radio Frequency Identification), in order to
realize information services through position-based communications
(see, for instance, Yoshiyuki Nakamura, Takuichi Nishimura, Hideo
Itoh, and Hideyuki Nakashima, "ID-CoBIT: A Battery-less Information
Terminal with Data Upload Capability", Proc. of the 29.sup.th
Annual Conference of the IEEE Industrial Electronics Society
(IECON), November 2003).
[0004] One of such applications is an information communication
service using a card-sized communication terminal (see, for
instance, National Institute of Advanced Industrial Science and
Technology, Information Technology Research Institute,
"Newsletter", ITRI@EXPO 2005 AICHI JAPAN, March 2005)
[0005] This communication terminal incorporates an IR-based spatial
optical communication system and an active-type wireless IC tag
system. In the former is realized battery-less capture of voice
information. In the latter, which involves a button battery-powered
radio source, is realized people-flow analysis in exhibition
grounds or the like.
SUMMARY
[0006] When IC tags are used for grasping the movement of mobile
bodies such as objects or people, however, if the frequency of the
radio waves generated by the IC tags exceeds 30 MHz, the influence
of local scattering in indoor measurements cannot be neglected,
(Katsumi Furuya, Tomoteru Kawakami, Hiroyoshi Yajima, "Study of
Microscopic Loop Antennas Measurement by a 3-Antenna Method", IEEE
Society of Instrumentation and Measurement, Japan Chapter, IM-01-3,
pp. 13-17, February 2001), and it becomes thus difficult to
estimate the position of the radio source using a purely far
field-based methodology.
[0007] That is, in light of the frequency bands of about 300 MHz
widely used at present, grasping position information in a closed
space based on an IC tag system alone requires solving the inverse
problem of estimating radio sources on the basis of electric field
intensity and/or phase as measured by receivers (Yoshio Okamoto,
"Inverse Problems and Solution Methods", Ohmsha Ltd., 1992).
[0008] Solving such a problem, however, is extremely difficult,
while calculating position information in real time by introducing
a process in which the Maxwell equations are solved on the basis of
boundary conditions that take into account local structures is
considered to be virtually impossible.
[0009] For this reason, grasping in real time position information
based on an IC tag system alone in indoor facilities such as event
halls, department stores or the like, in which there is a
substantial demand for information services, entailed hitherto
substantially higher costs for implementing a detection
environment, for instance through the arrangement of receiving
antennas in a number proportional to position density, and through
detection and estimation of entering and leaving in/from reception
areas by mobile objects of which IC tags are accessory.
[0010] For instance, when it comes to monitoring space disturbances
in areas and/or times in which no mobile objects must be present,
mainly in security services, judging whether there are any mobile
objects present or not is more important than grasping the movement
of the mobile objects. There are methods for achieving this goal
with plural receivers arranged as described above wherein, in
addition to high implementation costs, it may not be possible to
attach an IC tag to a mobile object. Specific examples include, for
instance, detection of intruders after office hours in companies,
department stores, vaults, storage facilities, military weapon
depots, or in private residences when no one is home or in the deep
night hours.
[0011] In light of the foregoing, it is an object of the invention
of the present application to provide a mobile object detection
method, and a mobile object detection system, that allow detecting
the movement of an object based on measurements of the electric
field intensity generated by transmitters such as IC tags, while
curbing implementation costs.
[0012] In order to solve the above problems, firstly, the mobile
object detection method of the invention of the present application
comprises the steps of measuring electric field intensity generated
by a plurality of transmitters arranged in a detection space;
obtaining a difference between the electric field intensity and an
average value of the electric field intensity in a state where an
object in the detection space is not moving; and detecting the
movement of the object when the difference is equal to or larger
than a threshold value based on a standard deviation of the
electric field intensity in a state where the object in the
detection space is not moving.
[0013] Also, the mobile object detection system of the invention of
the present application comprises means for measuring electric
field intensity generated by a plurality of transmitters arranged
in a detection space; and means for obtaining a difference between
the electric field intensity and an average value of the electric
field intensity in a state where an object in the detection space
is not moving, and detecting the movement of the object when the
difference is equal to or larger than a threshold value based on a
standard deviation of the electric field intensity in a state where
the object in the detection space is not moving.
[0014] In accordance with the first and second inventions of the
present application, mobile object detection can be realized while
curbing implementation costs by fixedly arranging beforehand in a
detection space transmitters such as active-type wireless IC tags
for sending signals, as a radio wave medium, without the
transmitters being attached to the mobile object, the electric
field intensity generated by the respective transmitters being
measured by one or relatively few receivers such as tag readers,
fixedly arranged separately in the detection space, and by, for
instance, centrally managing electric field intensity data with a
computer, sequentially calculating an average value and a standard
deviation of electric field intensity within a prescribed lapse of
time when no object is moving, observing electric field intensity
changes when the object moves, and noting divergences from the
average value equal to or larger than a threshold value based on
the standard deviation.
[0015] Mobile object detection can be realized, thus, simply by
monitoring the average value and standard deviation of the electric
field intensity generated by a plurality of transmitters arranged
at intervals, even in environments such as in rooms or in corridors
where, on account of multiple reflection and/or interference, the
electric field intensity formulae based on far-field are
ineffective and estimation of transmitter positions is not
possible. That is, movement of an object can be detected, even in
closed spaces where any obstacle may be present, by arranging
transmitters at locations that allow the receivers to measure
electric field intensity on a regular basis.
[0016] The invention of the present application can be suitably
used for a wide variety of applications, obviously in
position-based information services, but also in the field of
security services, being effective in all manner of services
relating to monitoring of space disturbances in areas and/or times
in which no mobile objects must be present. Specifically, the
invention can be suitably used for implementing inexpensive
detection of intruders after office hours in companies, department
stores, vaults, storage facilities, military weapon depots, or in
privates residences when no one is home or in the deep night hours,
detection of piping anomalies in nuclear facilities or the like, or
detection of abnormal situations in underground passages or
tunnels.
DESCRIPTION OF DRAWINGS
[0017] FIG. 1 is a diagram explaining the invention of the present
application;
[0018] FIG. 2 is a diagram illustrating a measurement example of
electric field intensity in the situation of FIG. 1;
[0019] FIG. 3 is a diagram explaining the invention of the present
application;
[0020] FIG. 4 is a diagram illustrating a measurement example of
electric field intensity in the situation of FIG. 3;
[0021] FIG. 5 is a diagram illustrating an example of the invention
of the present application;
[0022] FIGS. 6A and 6B are diagrams illustrating an example of the
invention of the present application;
[0023] FIGS. 7A and 7B are diagrams illustrating an example of the
invention of the present application;
[0024] FIGS. 8A and 8B are diagrams illustrating an example of the
invention of the present application; and
[0025] FIG. 9 is a diagram illustrating an example of the invention
of the present application.
DETAILED DESCRIPTION
[0026] Firstly, a plurality of transmitters are fixedly arranged at
equidistant intervals such that, as explained next, the temporal
change of electric field intensity when an object is moving is
large enough to be sufficiently distinguished from the temporal
change of electric field intensity when the object is not
moving.
[0027] In situations where local scattering can be neglected,
electric field intensity measured by a receiver is given by the far
field-based formula below when the distance between a transmitter
such as an active-type IC tag and a receiver such as a tag reader
is sufficiently larger than the wavelength (Saburo Adachi,
"Electromagnetic Wave Engineering", Corona Publishing Co. Ltd., p.
39, 1983) E .function. ( r ) = C 0 R Formula .times. .times. 1
##EQU1##
[0028] In the formula, E represents the electric field intensity, R
the distance between transmitter and receiver, and C.sub.0 a
constant related to transmission power and reception
efficiency.
[0029] When local scattering cannot be neglected, the Maxwell
equations must be solved on the basis of boundary conditions that
take into account local structures, in order to obtain an
expression corresponding to Formula 1 that links electric field
intensity and distance.
[0030] When the wave source is moving, however, such a relational
expression changes over time, and hence although a method could be
devised for solving the Maxwell equations sequentially, comparing
the measured electrical fields, and estimating the position of the
wave source, solving in real time such a problem for a closed space
lacking structural symmetry would be extremely difficult at present
for the capabilities of ordinary computers.
[0031] In the invention of the present application, therefore, in
order to detect the movement of an object without solving
sequentially the Maxwell equations, the average value and standard
deviation of electric field intensity when the object is not moving
are monitored; the change in electric field intensity when the
object moves is observed; deviances equal to or larger than a
threshold value based on the standard deviation is noted; and,
hence, the movement of the object is detected.
[0032] In environments with large local scattering, plural
transmitters n (=1, . . . , N) are fixedly arranged spaced from one
another, and their respective electric field intensity is
continually measured by receivers. If the energy supplied by the
transmitters is constant, the time-series data of the electric
field intensity measured by the respective receivers, in the
absence of disturbances such as the movement of an object, exhibit
a distribution similar to a normal distribution centered around the
average value E.sub.average of the time series data of the nearest
past m time series data, without deviating substantially from
E.sub.average. A normal distribution refers herein to a
distribution in which 99.7% of all data fall within the
average.+-.3 standard deviations, for instance, taking .DELTA.E
(=threshold value) as 10 times the standard deviation, virtually
100% of the electric field intensities En of the transmitters n
measured by the receivers falls within the range below.
E.sub.average-.DELTA.E<E.sub.n<E.sub.average+.DELTA.E Formula
2
[0033] The nearest past m time series data are the number of data
acquired over a period of time sufficiently longer than the time
elapsed from the moment the object switches from a moving state to
a stationary state until the value of the electric field intensity
becomes substantially constant. In the case of a pedestrian as the
mobile object, the field becomes substantially constant in a matter
of several seconds, and hence m becomes about 100 (corresponding to
5 seconds) if the signal emission frequency of the transmitters is
set to, for instance, 0.05 seconds.
[0034] The "standard deviation" is a value obtained by taking the
square root of the average of the squares of the differences
between the average value E.sub.average and the respective measured
values.
[0035] When in such an environment the object is large compared to
the wavelength size of the radio waves of the transmitters, the
movement of the object affects the electric field intensity, which
thereupon deviates widely from the reference value E.sub.average.
The movement of the object is detected based on the extent of such
deviation. That is, the object is judged to be moving if the
difference between a measured value E.sub.n and the average value
E.sub.average is sufficiently larger than, for instance, .DELTA.E,
which is 10 times the standard deviation (i.e., if the inequality
of formula 2 is not satisfied).
[0036] Although the threshold value .DELTA.E was taken as 10 times
the standard deviation in the above explanation, the threshold
value .DELTA.E is not particularly limited thereto, and may be
arbitrarily set so as to afford a sufficiently precise detection in
accordance with factors such as the shape and size of the detection
space, the type of transmitters and receivers, and the arrangement
relationship thereof.
[0037] The above detection method is explained in detail on the
basis of concrete embodiments illustrated in FIGS. 1 through 4.
[0038] Firstly, transmitters 1 to 52 are fixedly arranged at
equidistant intervals within a triangular planar area such as the
one illustrated in FIG. 1, while receivers 1 to 3 are arranged
fixedly at the vertices of the triangular area surrounding the
transmitters 1 to 52; and, the receivers 1 to 3 are continually
measuring the electric field intensity generated by the
transmitters 1 to 52.
[0039] An obstacle crosses then the north side of the triangular
area (between the line joining the transmitters 1 and 2 and the
line joining the transmitters 3, 4 and 5), as illustrated in FIG.
1.
[0040] In such a case, the radio waves for the electric field
intensity of a transmitter n (=1,2) measured by the receiver 1 are
not blocked when the obstacle is crossing, and hence the electric
field intensity keeps within a range smaller than a change to the
extent of .DELTA.E.sub.n,1 based on the standard deviation. By
contrast, the radio waves for the electric field intensity of a
transmitter n (=3, 4, 5, . . . , 52) measured by the receiver 1 are
blocked when the obstacle is crossing, and hence the electric field
intensity exhibits a change that deviates from .DELTA.E.sub.n,1. As
illustrated in FIG. 2, specifically, the electric field intensity
E.sub.4,1 of the transmitter 4, the electric field intensity
E.sub.12,1 of the transmitter 12, and the electric field intensity
E.sub.24,1 of the transmitter 24, exhibit only fluctuation changes
when the obstacle is not crossing, and the relationship of the
above Formula 2 holds; at the moment in which the obstacle is
crossing, however, the electric field intensities change beyond
fluctuations and acquire values largely diverging from the
respective standard deviations .DELTA.E.sub.4,1, .DELTA.E.sub.12,1
and .DELTA.E.sub.24,1. That is because the radio waves that ought
to reach the receiver 1 are blocked by the obstacle, which causes
the measured value of the electric field intensity to drop
steeply.
[0041] Meanwhile, the electric field intensity E.sub.n,2 of a
transmitter n (=1,2) measured by the receiver 2 exhibits similarly
a change equal to or larger than .DELTA.E.sub.n,2, while the
electric field intensity E.sub.n,2 of a transmitter n (=3, 4, 5, .
. . , 52) does not exhibit a change equal to or larger than
.DELTA.E.sub.n,2, since the obstacle does not block the radio waves
between the transmitters and the receiver. The same applies to the
receiver 3.
[0042] The time series data of the electric field intensities of
the transmitters n (1, 2, . . . , 52) acquired by the plural
receivers 1 to 3 are processed by a network, a computer or the
like, to allow identifying the transmitters entering a blind spot
due to the obstacle, and to grasp the travel made by the obstacle,
or whether the obstacle is moving.
[0043] Next, an obstacle falls hypothetically on a point in the
middle of a line joining the transmitters 6, 7, 8 and 9 within the
planar area, as exemplified in FIG. 3.
[0044] In such a case, when the radio waves for the electric field
intensity of a transmitter n (=1 to 52) measured by the receiver 1
are not blocked by the obstacle, the electric field intensity keeps
within a range smaller than a change to the extent of the value
.DELTA.E.sub.n,1 based on the standard deviation, while when the
radio waves are blocked by the obstacle, the electric field
intensity exhibits a change that deviates from .DELTA.E.sub.n,1. As
illustrated in FIG. 4, specifically, the various electric field
intensities E.sub.4,1, E.sub.12,1 and E.sub.24,1 exhibit only
fluctuation changes when the obstacle has not fallen yet, and the
relationship of the above Formula 2 holds; at the moment in which
the obstacle falls, however, the radio waves become partially
blocked, and thenceforth the electric field intensities E.sub.12,1
and E.sub.24,1 acquire values largely diverging from the respective
standard deviations .DELTA.E.sub.12,1 and .DELTA.E.sub.24,1 Over
time, a new constant value sets in, and fluctuations become again
approximately equivalent to the standard deviation.
[0045] In the receiver 2, meanwhile, the obstacle blocks radio
waves only for the electric field intensity E.sub.5,2 of a
transmitter (for instance, n=5), whereby the electric field
intensity E.sub.5,2 drops deviating from .DELTA.E.sub.5,2, while in
the receiver 3 the obstacle blocks radio waves only for the
electric field intensity E.sub.3,3 of a transmitter (for instance,
n=3), whereby the electric field intensity E.sub.3,3 drops
deviating from .DELTA.E.sub.3,3.
[0046] The time series data of the electric field intensity
acquired by the receivers 1 to 3 are comprehensively processed by a
network or the like, to allow identifying the transmitters entering
a blind spot due to the obstacle, and to estimate the time at which
the object fell and the spot on which it fell.
EXAMPLE 1
[0047] 1. Experiment for Measuring Electric Field Intensity
[0048] An experiment carried out to measure actual electric field
intensities is explained below.
[0049] The equipment used in the experiment, comprising:
[0050] Receiver, Tag reader LAS300R (from K-ubique ID Co.); and
[0051] Transmitters, Active RFID tag LA300T1 (from K-ubique ID
Co.).
[0052] The measurement results were analyzed employing an analysis
program created using Lab VIEW (from National Instruments); this
program was installed in a computer in which was executed the
analysis process for the input measured values of the tag
reader.
[0053] The radio waves of the tags had a frequency of 315 MHz, a
wavelength of about 1 m, and the experimental environment was a
frequency area in which local scattering could not be neglected
(see Katsumi Furuya, Tomoteru Kawakami, Hiroyoshi Yajima, "Study of
Microscopic Loop Antennas Measurement by a 3-Antenna Method", IEEE
Society of Instrumentation and Measurement, Japan Chapter, IM-01-3,
pp. 13-17, February 2001).
[0054] To build the detection system of the invention of the
present application are used, for instance, means for detecting
electric field intensity, and, as mobile object detection means, a
receptor such as a tag reader mapped to a transmitter, and an
analysis program and/or a device such as a computer in which is
installed such a program, in which case the analysis program has a
function not only for analyzing the average value and standard
deviation of electric field intensities but also a detection
program function for monitoring such average values and standard
deviations and for determining threshold values.
[0055] 2. Electric Field Intensity Dependence on
Transmitter-Receiver Distance
[0056] In order to study first whether far-field conditions
indicated by Formula 1 apply effectively indoors, a tag reader was
installed and a tag was positioned at a distance r from the tag
reader, then the electric field intensity was measured.
[0057] FIG. 5 illustrates the distance dependence of electric field
intensity measured by the tag reader in a corridor (width 2.4 m,
height 2.7 m) with no people traffic. The x-axis represents the
distance r (m) between tag and reader, and the y-axis represents
the electric field intensity E (arbitrary units). As FIG. 5 shows,
the distance dependence of electric field intensity does not
conform to Formula 1, which means that local influences cannot be
neglected.
[0058] 3. Mobile Object Detection Based on Divergence of the
Electric Field Intensity Average Value from the Standard
Deviation
[0059] Mobile object detection is explained next.
[0060] 3.1 In Absence of Object Movement (=Steady State)
[0061] Firstly, as illustrated in FIG. 6A, a reader was installed
and three tags A5, A6 and A7 were arranged consecutively away from
the reader at intervals of 4 m, then electric field intensity was
measured in a state of no object movement. The tag reader, tags,
analysis (detection) program were identical to those described
above.
[0062] FIG. 6B illustrates the temporal change of electric field
intensity. The x-axis represents herein time (seconds) and the
y-axis represents the electric field intensity E of the tags A5, A6
and A7, sequentially from the bottom, in arbitrary units. The
average value and the standard deviation of the electric field
intensities of the tags A5, A6 and A7 are given in Table 1.
TABLE-US-00001 TABLE 1 Electric field intensity average value
Standard deviation Tag number E.sub.average [A.U.] .DELTA.E A5
170.1 0.379 A6 163.1 0.431 A7 134.8 0.586
[0063] As Table 1 shows, the temporal change of electric field
intensity, in a steady state with no object movement, involves a
standard deviation of 0.5% or less in environments in which local
scattering cannot be neglected. This tendency is independent from
the distance between tag and reader (between transmitter and
receiver).
[0064] 3.2 Upon Object Movement
[0065] Mobile object detection is approached on the basis of the
above facts.
[0066] As illustrated in FIG. 7A, a reader was installed and three
tags A5, A6 and A7 were arranged consecutively away from the reader
at intervals of 6 m, then an object having a size similar to the
wavelength of the tags, of about 1 m, moved in a straight line for
several seconds, following the path below, at intervals of 20
seconds. The tag reader, tags, analysis (detection) program were
identical to those described above.
[0067] (Rear of the tag reader).fwdarw.(between tag reader and
A5).fwdarw.(between A5 and A6).fwdarw.(between A6 and A7)
.fwdarw.(ahead of A7).fwdarw.(between A7 and A6).fwdarw.(between A6
and A5) .fwdarw.(between A5 and tag reader) .fwdarw.(rear of the
tag reader).
[0068] FIG. 7B illustrates the temporal change of electric field
intensity. The x-axis represents herein time (seconds) and the
y-axis represents the electric field intensity E of the tags A5, A6
and A7, sequentially from the bottom, in arbitrary units. As FIG.
7B shows, the electric field intensity change at the 20 second
intervals, when the object moves, deviates from the steady state
average value E.sub.average by more than .DELTA.E, which is 10
times the standard deviation.
[0069] In a more detailed analysis, the object starts moving at the
20 second mark in FIG. 7B, passes the tag reader, and stops between
the tag reader and the tag A5. At this time, the average value
E.sub.average of the electric field intensity drops for all the
tags A5, A6 and A7, by more than .DELTA.E, which is 10 times the
standard deviation in the steady state. This drop results from
blocking of the radio waves of the tags A5, A6 and A7 by the
object.
[0070] At the 40 second mark, the object passes the tag A5 and
stops between the tag A5 and the tag A6. At this time, the average
value E.sub.average of the electric field intensity of the tag A5
rises by more than .DELTA.E, which is 10 times the standard
deviation, but the average value E.sub.average of the tags A6 and
A7 does not change by that much. This indicates that the object
does not block now radio waves between the tag reader and the tag
A5.
[0071] At the 60 second mark, the object passes the tag A6 and
stops between the tag A6 and the tag A7. At this time, the average
value E.sub.average of the electric field intensity of the tag A6
rises by more than .DELTA.E, which is 10 times the standard
deviation. This indicates that the object does not block now radio
waves between the tag reader and the tag A6.
[0072] At the 80 second mark, the object passes the tag A7 and
stops ahead of the tag A7. At this time, the average value
E.sub.average of the electric field intensity of the tag A7 rises
by more than .DELTA.E, which is 10 times the standard deviation.
This indicates that the object does not block now radio waves
between the tag reader and the tag A7.
[0073] At the 100 second mark, the object turns around from its
position ahead of the tag A7 and stops between the tag A7 and the
tag A6. At this time, the average value E.sub.average of the
electric field intensity of the tag A7 drops by more than .DELTA.E,
which is 10 times the standard deviation. This indicates that the
object blocks again radio waves between the tag reader and the tag
A7.
[0074] The change in electric field intensity on account of object
movement every 20 seconds thereafter can be explained as above.
[0075] From the foregoing it follows that the movement of an object
can be detected by monitoring the average value E.sub.average and
standard deviation .DELTA.E/10 of the electric field intensity in
the absence of object movement, and by noting divergences from the
average value E.sub.average by .DELTA.E or more.
[0076] Such movement detection is effective for detecting abnormal
situations in closed spaces that are ordinarily unmanned in, for
instance, department stores, vaults or the like after office hours,
or in nuclear facilities and the like.
EXAMPLE 2
[0077] Detection of Door Opening/Closing
[0078] Herein was detected the opening and closing of a door and
the entry and exit of a mobile object into and out of a room, to
confirm the feasibility of abnormal-state detection. The tag
reader, tags and the analysis (detection) program were the same as
above.
[0079] As illustrated in FIGS. 8A and 9, a tag reader was installed
in a corridor in front of a door, and tags were arranged in the
corridor at intervals of 2 m. The door was opened 20 seconds after
the start of the experiment ("start" in the figure); 40 seconds
after experiment start an object came out of a room and passed the
tags A7 to A9; 60 seconds after experiment start the object passed
again the tags A7 to A9 and entered into the room; and 80 seconds
after experiment start the door was closed.
[0080] FIG. 8B illustrates temporal changes of electric field
intensity, in which the electric field intensity E of the tags A5
to A9 is represented sequentially from the top in arbitrary units.
The x-axis represents herein time (seconds) and the y-axis
represents the electric field intensity E.
[0081] As FIG. 8B shows, large electric field intensity changes
occur in the vicinity of the time marks when the object moves every
20 seconds. These changes, which largely exceed the .DELTA.E value
that is 10 times the standard deviation of the ordinary electric
field intensity, are the basis of the mobile object detection
method. Also, the door remains open between the 20 second mark and
the 80 second mark, during which the time average value
E.sub.average of the electric field intensity of the tags A6 and A7
close to the tag reader rises considerably above that of other
tags. This allows assessing the open state of the door.
[0082] As made clear by way of the above examples, the method for
detecting the movement of an object according to the invention of
the present application allows detecting the movement and the
position of a mobile object to be detected by measuring the
electric field intensity generated by transmitters, even when no
transmitter is attached to the mobile object.
[0083] In environments where it is not possible to estimate
transmitter position on a far-field basis, the movement of an
object can also be detected by fixedly arranging transmitters at
equidistant intervals, measuring the respective electric field
intensities and sequentially calculating the average value and the
standard deviation of electric field intensity within a prescribed
lapse of time, observing electric field intensity changes when the
object moves, and noting such divergences from the average value
that are equal to or larger than a threshold value based on the
standard deviation.
[0084] The arrangement relationship among the transmitters and the
receivers is not particularly limited, and for instance the
arrangements exemplified in FIGS. 1 and 3, and/or the arrangements
described in the above examples can be arbitrarily designed in
accordance with the actual environment, provided that the
arrangement relationship allows the receivers to measure on a
regular basis the electric field intensity generated by the
transmitters.
[0085] As described above, the invention of the present application
can be suitably used for a wide variety of applications not only in
position-based information services but also in the field of
security services, being effective in all manner of services
relating to monitoring of space disturbances in areas and/or times
in which no mobile objects must be present. Specifically, the
invention can find suitable application, among others, in the
detection of intruders after office hours in companies, department
stores, vaults, storage facilities, military weapon depots, or in
privates residences when no one is home or in the deep night hours,
detection of piping anomalies in nuclear facilities or the like, or
detection of abnormal situations in underground passages or
tunnels. Moreover, these service systems can be built
inexpensively.
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